Automatic transmissions utilize one or more mechanically actuated one-way clutches, such as sprags or roller clutches, to eliminate potential interference during a shift interchange. The one-way clutches function similar to a very stiff spring in the forward direction and provide a step change in the effective spring rate which changes abruptly to a zero spring rate with a small amount of drag torque when the direction of torque transmission is reversed. These one-way clutches do not exhibit any perceptible delay in the apply or release conditions.
Hydraulic one-way clutches have been proposed for a number of years. There are a variety of ways of executing a hydraulic one-way clutch, but the most common is to use rotational lash valves to switch piston apply pressure on or off, depending upon the relative direction of torque. These devices have an advantage over the mechanical one-way clutch in that the apply pressure can be modulated to dissipate energy on apply or release and can provide two-way torque transmission when desired.
The prior art transmissions utilizing one-way clutches frequently place a conventional friction device in series with the one-way device to provide for engine braking during coast operation.
The most obvious disadvantage of existing one-way clutches is the potential delay for apply or release after or when the torque direction is reversed. The mechanical one-way devices only experience the delay of changing from a very stiff spring rate to a zero spring rate which is very slight. The hydraulic one-way clutches need time to flow oil in or out of the clutch chamber which has a potential to noticeably change the quality of the shift in an automatic automotive transmission.
It has been found with hydraulic one-way clutches that the shift delay is most noticeable during a downshift; that is, a shift interchange from a high ratio to a low ratio. Prior art patents which disclose hydraulic one-way clutches are U.S. Pat. No. 4,805,752 issued to John D. Malloy on Feb. 21, 1989, or U.S. Pat. No. 4,969,546 issued to Raymond J. Haka on Nov. 13, 1990.